1. Field of the Invention
The present invention relates to lens meters capable of performing a lens-characteristic mapping display of a subject lens to be tested.
2. Description of the Related Art
In conventional lens meters, a patterning plate 50 with circular holes serving as patterns 50a for measuring the lens characteristics of a subject lens is provided in the optical path of a measuring light beam emitted by a light source portion, as shown in FIG. 17. For example, a progressive power lens as a subject lens is set in the optical path, and the measuring light beam is projected onto the progressive power lens set in the optical path so that a great number of lens-characteristic measuring pattern images are received on a charge-coupled device (CCD). The images received are analyzed, and the lens characteristics, such as a spherical degree S (see FIG. 18(a)), a cylindrical degree C (see FIG. 18(b)), an axial angle A (see FIG. 18(c)), and a prism degree Prs (see FIG. 18(d)), are displayed on a display screen (mapping display).
In this kind of lens meter, a center-position determining pattern or position specifying pattern 51 consisting of four slits 50b through 50e and presenting a square shape as a whole is formed in the central portion of the patterning plate 50 so that the center position of the pattern image corresponding to the center position Oo of the patterning plate 50 can be determined, even when the optical axis of a subject lens is offset from the optical axis of the optical path.
The lens meter of the above kind is equipped with a means for determining the center position of the pattern image corresponding to the center position Oo of the patterning plate 50. The determining means processes a great number of pattern images received by the CCD, thereby identifying the difference in configuration between the image of the lens-characteristic measuring pattern 50a and the center-position determining pattern 51 and extracting the image of the center-position determining pattern 51. Based on the extracted image of the center-position determining pattern 51, the center position of the pattern image corresponding to the center position Oo of the patterning plate 50 is determined.
However, about 1000 lens-characteristic measuring patterns 50a are provided in the patterning plate 50 and, therefore, in order to determine the center position of the pattern image corresponding to the center position Oo of the patterning plate 50, the images of about 1000 lens-characteristic measuring patterns 50a and the image of the center-position determining pattern 51 must be processed. For this reason, there are the disadvantages that the image processing will take time, and that the center position of the pattern image corresponding to the center position Oo of the patterning plate 50 cannot be quickly determined.
In particular, when a subject lens set in the optical path is moved in a direction crossing the optical path, the image information being displayed on the display screen is difficult to move quickly so as to follow the movement of the subject lens.
In another lens meter of the above kind, a great number of lens-characteristic measuring patterns 50a are provided in a patterning plate 50, and the lens-characteristic measuring pattern 50a corresponding to the center position Oo of the patterning plate 50 is omitted. Based on the omission information on the image of the lens-characteristic measuring pattern 50a corresponding to the center position Oo of the patterning plate 50, the center position of the pattern image corresponding to the center position Oo of the patterning plate 50 is determined.
However, in the case of the lens meter which omits the lens-characteristic measuring pattern 50a which should be located at the center position Oo of the patterning plate 50, if the patterning plate 50 becomes dirty and even a single pattern of the lens-characteristic measuring patterns 50a which should be there is omitted, then the places at which the images of the lens-characteristic measuring patterns 50a are omitted will be 2 or more. Consequently, there is the disadvantage that the center position of the pattern image corresponding to the center position Oo of the patterning plate 50 cannot be determined.
In the case of an unprocessed lens with paint marks, a measuring light beam is shut out at a position at which the marks are painted. Therefore, as with the aforementioned case, the places at which the images of the lens-characteristic measuring patterns 50a are omitted will be 2 or more, and consequently, the center position of the pattern image corresponding to the center position Oo of the patterning plate 50 cannot be determined.